1. Field of the Invention
The present invention relates to an air heater, and particularly to an air heater for heating intake air for an internal combustion engine of the type incorporated at the entrance or at a middle portion of the air intake passage of the engine for heating intake air during cold starting or further warming up of the engine.
2. Description of the Prior Art
It is known to employ an air heater in order to improve cold starting or further warming up performance of an internal combustion engine, particularly an engine of the diesel type. It is also known to employ, as a heating element of such an air heater, an element having a ceramic body of a perforated structure, selected surface portions of which are applied with electro-conductive layers which operate as electrodes, generally referred to as a PTC heater, as it is generally made to show a positive thermal coefficient. The ceramic material for such a purpose has high configurational flexibility as compared with metallic materials for heater elements and can be readily formed into a perforated plate having a lattice structure or a honeycomb structure which provides a large heating surface which increases more as the mesh of the lattice or honeycomb structure becomes finer.
It is more desirable from the view point of facilitating cold starting and further warming up of the engine to increase the heating surface area of such a heater element, while on the other hand the increased fineness of the mesh also increases the flow resistance applied to the intake air conducted therethrough, thereby increasing the pressure drop in the intake system of the engine, resulting in the droppage of the engine output power. Therefore, in the intake air heater of the type which employs a heater element in the form of a perforated plate such as a lattice plate or a honeycomb plate which traverses the intake air passage, a compromise has been conventionally made with regard to the balance between the air heating capacity available by the heater element and the pressure drop incurred thereby. In this case, in the conventional heater structure wherein the perforated plate heater element traverses the entire cross section of the intake passage of the engine, the above-mentioned compromise was only unilaterally conditioned by the characteristics of the heating capacity which monotonously increases according to increase of the fineness of the lattice or honeycomb structure and the characteristics of the flow resistance which also monotonously increases according to increase of the fineness of the lattice or honeycomb structure.